1. Field of the Invention
The invention concerns re-engaging an automatically controlled friction clutch, in particular for an automobile vehicle equipped with an automatic or semi-automatic transmission.
2. Description of the Prior Art
The invention is more precisely concerned with a method of automatically operating a clutch release bearing during the re-engagement phase following a change in gear ratio, the clutch release bearing operating on a clutch release member of the clutch in question, in accordance with predetermined laws of displacement speed conditioned by the rotation speeds of two shafts coupled in rotation by the clutch.
As is well known, a friction clutch is a device for transmitting frictionally, between a first shaft and a second shaft coaxial with the first, a torque which is modulated according to the operation of an associated clutch release bearing on a clutch release member of the clutch. The clutch release bearing has a predetermined total travel between a clutch released position in which no torque is transmitted between the shafts and a clutch engaged position in which the shafts are coupled together in rotation. This total travel generally comprises a dead travel between the clutch released position and an intermediate position, sometimes called the take-up position (in which torque begins to be transmitted from one of the shafts to the other), a usable travel corresponding to increasingly complete transmission of torque up to a complete coupling position, and a final travel to the clutch engaged position which serves to consolidate the rotational coupling of the shafts.
In more precise terms, a friction clutch of this kind comprises a group of at least two transverse plates rotationally coupled to the first shaft but free to move closer together or further apart, at least one friction disk fastened to the second shaft disposed between the plates, and the aforementioned clutch release member, such as a diaphragm, normally urging the plates towards each other so as to frictionally lock together the plates and the friction disk, but which can release its pressure continuously in response to the action of the clutch release bearing, so as to permit progressive decoupling of the two shafts. This decoupling is accompanied by circumferential slipping between the plates and friction disk, which is why the term slip travel or area is sometimes used for the travel of the clutch release bearing between its take-up position and its complete coupling or even its clutch engaged position.
In an automatic clutch control system the problem arises of defining the displacement of the clutch release bearing between its extreme positions. A clutch of this kind is generally associated with a gearbox and it is necessary to implement automatically transient conditions that are as smooth and flexible as possible on changing the gearbox ratio.
The invention is more specifically concerned with engaging a new ratio within a gearbox, that is to say, in more general terms, with coupling together again two shafts rotating at sufficient and non-zero speeds.
One such determination of the conditions for re-engaging a clutch between two shafts rotating at non-zero speeds is proposed, for example, in French Pat. No. 2 525 163 (patent application Ser. No. 83 06708 dated Apr. 19, 1983); this document teaches that, on changing gear ratio, the clutch release bearing is displaced over its slip travel (up to its clutch engaged position, nothing being mentioned with regard to the approach travel from the clutch released position) at a speed directly proportional to the degree to which the motor speed is greater than the speed of the gearbox input shaft and, preferably, the transmission ratio selected in the gearbox. This document therefore teaches the provision of a number of programs covering the slip travel of which one is selected for the re-engagement phase according to the relative rotation speeds of the shafts.
However, it has been found that this solution is not fully satisfactory from the point of view of the smoothness and flexibility of driver control in the case of an automobile vehicle, and in that it has been found necessary to carry out experimental tests in order to define the speed programs to be implemented for each individual application.
An object of the invention is to provide for increased flexibility of operation and simplified determination of the speed programs to be used.